Diagnosing Embedded Mixed-Phase Layers Associated with Rimed Snow Using Doppler Radars

Layers of supercooled liquid cloud are frequently observed in polar and extratropical atmospheres, but often elude detection when embedded within ice clouds, where ice particles dominate radar backscatter and lidar is quickly extinguished. In some cases the presence of these elusive embedded mixed-phase layers may be inferred from the riming of precipitating ice particles below. The mass content of rimed snow and the distribution and persistence of mixed-phase clouds are important components of the global hydrological cycle, the remote-sensing and numerical representation of which will require an improved understanding of the microphysical processes at work in mixed-phase clouds.

In this study we demonstrate the novel retrieval of the degree of riming of snow from ground-based Doppler radars from the Biogenic Aerosols---Effects of Clouds and Climate (BAECC 2014) field campaign at Hyytiala, Finland, and evaluate the retrievals against in situ particle imaging measurements. Estimates of the properties of snow through the vertical profile reveal that the onset of riming is associated with increased ice water content due to the accretion of supercooled liquid droplets, and can be diagnosed by an increase in Doppler velocity. Hence the retrieval of rimed snow can be used to diagnose embedded layers of supercooled liquid within optically thick ice clouds. Using the detection of embedded mixed-phase cloud to update an existing radar/lidar synergy cloud classification scheme, we discuss the potential for an improved global climatology of mixed-phase cloud from the upcoming ESA/JAXA EarthCARE satellite, which will include the first Doppler cloud profiling radar in space.